It is known that the insoluble anodes for use in the electrowinning of cells for heavy metal production, such as, e.g., lead, zinc and lead, impose the use of semifinished pieces having values of cross-section surface area and of mechanical strength adequate for conducting electrical currents of considerable intensity, and suitable for building strong structures, which are capable of being not deformed by possible impacts, and of maintaining a precise position inside the cell.
Substantially, it is necessary that such insoluble anodes, owing to reasons of electrical current conduction, as well as reasons of mechanical strength, have a good firmness, as well as a certain weight.
While keeping into due account said basic requirement, for a long time those skilled in the art have been looking for an ideal material for manufacturing insoluble anodes, which also makes it possible above all characteristics of long useful life to be achieved in the anode, even under severe operating conditions. Therefore, such search is directed towards materials which, while being endowed with an at least rather good electrical conductivity and mechanical strength, also simultaneously display a high chemical inertness towards the more aggressive and corrosive agents.
In this connection, tantalum, niobium and titanium, metals endowed with good ductility and malleability, as well as with a rather good heat and electrical conductivity, are appreciated above all thanks to their chemical inertness towards the most aggressive media.
Tantalum, in particular, by getting coated by an extremely thin oxide layer, becomes resistant to nearly all reactants, at temperatures of up to 200.degree.-300.degree. C. Only hydrofluoric acid, fluorides, hot concentrated alkalies and sulphur trioxide are capable of attacking such an oxide, and then the same metal.
However, such a rare metal is known to have a very high cost.
From the viewpoint of chemical resistance, having available insoluble tantalum anodes would represent the ideal solution. But, as we saw, such anodes should also comply with such requirements of firmness, weight and cross-section surface-area, as to supply very good guarantees of mechanical strength and of electrical conductivity, so that for that purpose insoluble tantalum anodes should be manufactured, having so large values of weight and dimensions, as to make the manufacturing thereof impracticable at an industrial level, owing to the extremely high cost of such a metal.
The same problem substantially exists for niobium and titanium too.